This investigation is concerned with development of methods for the determination of several vitamins of the B group and enzyme activity dependent on these vitamins in leukocytes as indicators of vitamin nutriture. In addition, the effects of modifications in diet and of anti-vitamins on the vitamin content and enzyme activity in leukocytes will be determined in rats. Investigations will be continued with leukocyte thiamine and transketolase activity. In addition, other experiments will be concerned with comparison of aspartate aminotransferase activity in leukocytes and in liver of animals on varied dietary intakes of vitamin B6. Similar experiments will be undertaken in rats with determination of glutathione reductase and xanthine oxidase activity of leukocytes when the diet provides varying quantities of riboflavin.